This invention relates to an alloy steel for fabrication into roll caster shells used in the direct casting of molten aluminum to sheet, and to a roll caster shell made therefrom. The steel of this invention exhibits markedly longer service life than the standard prior art steel used for this purpose.
Molten aluminum is direct cast at a temperature of about 675.degree. C. (1250.degree. F.) to sheet thickness between pairs of water cooled roll caster shells. After a number of hours of operation conventional roll shell surfaces develop heat checks or cracks which gradually penetrate deeper into the shell, causing marks on the cast strip and eventually breaking the shell. Periodic shutdowns are thus necessary in order to remachine the shell surfaces and eliminate the cracks. Each remachining may remove about 15% of the thickness of the shell, and eventually the shells must be scrapped after production of about 5 to 8 million pounds of aluminum, under present practice. Typically a shell made from the standard alloy steel operates for about 300 to about 500 hours before the first surface remachining (representing about 1 million pounds of aluminum), which reduces the shell to about 85% of its original thickness. In contrast to this, the steel of the present invention can operate for about 1500 to 2000 hours (representing about 4.5 million pounds of aluminum) before the first remachining, and requires reduction only to about 95% of its original thickness. At least about 10 million pounds of sheet aluminum can be produced with the roll caster shells of the present invention before scrapping thereof.
The standard alloy steel now used for roll caster shells comprises, in weight percent, from 0.53% to 0.58% carbon, 0.45% to 0.65% manganese, 0.20% to 0.30% silicon, about 0.02% maximum phosphorus, about 0.02% maximum sulfur, 0.40% to 0.50% nickel, 1.0% to 1.2% chromium, 0.45% to 0.55% molybdenum, 0.10% to 0.15% vanadium and balance essentially iron.
The standard aluminum die casting die steel now used, designated as H-13, comprises, in weight percent, from 0.30% to 0.40% carbon, 0.20% to 0.40% manganese, 0.80% to 1.20% silicon, 4.75% to 5.50% chromium, 1.25% to 1.75% molybdenum, 0.80% to 1.20% vanadium, and balance essentially iron. This alloy can be used for roll caster shells, but it is expensive and can present processing difficulties.
Literature references relating to thermal fatigue, thermal cracking, high temperature alloys and alloying elements are as follows:
Glenny, E., "Thermal Fatigue," Metallurgical Reviews, 1961, Vol. 6, No. 24. PA0 Sobolev, N. D. and Egoror, V. I., "Thermal Fatigue and Thermal Shock," Strength and Deformation in Non-uniform Temperature Fields, Edited by Ya. B. Fridman, Consultants Bureau, New York, 1964. PA0 High Temperature High Strength Alloys, AISI Publication, No. 601, New York, February, 1963. PA0 Benedyk, J. C. et. al., "Thermal Fatigue Behavior of Die Materials for Al Die Casting," Paper 111, 6th SDCE International Die Casting Congress, Cleveland, Ohio, Nov. 16-19, 1970. PA0 Young, W., "Are You Getting Maximum Performance From Your Die Casting Dies?" ASTME Paper, CM 68-587, 1968. PA0 Northcott, L., and Baron, H. G., "The Craze-Cracking of Metals," Jnl. of ISI, Dec. 1956. PA0 Glenny, E., "Thermal Fatigue Resistance of Martensitic Steels," Jnl. of Matls., JMLSA, Vol. 4, No. 1, March 1969. PA0 Rostoker, W., "Thermal Fatigue Resistance of Martensitic Steels," Jnl. of Matls., JMLSA, Vol. 4, No. 1, March 1969. PA0 Bain, E. C., and Paxton, H. W., Alloying Elements in Steel, ASM Publ., 1966. PA0 Archer, R. S., et. al., Molybdenum, Steels-Irons-Alloys, Climax Molybdenum Co. Publ., N.Y., 1962. PA0 Vanadium, Steels and Irons, Vanadium Corp. of America, N.Y., 1937.